1. Technical Field
The present invention relates to clock generation in general, and, in particular, to a method and apparatus for generating spread spectrum clock signals. Still more particularly, the present invention relates to a method and apparatus for generating half-period spread spectrum clock signals having harmonic emission suppressions.
2. Description of Related Art
Many electronic devices employ processors and/or other digital circuits that require clock signals for synchronization. Clock signals can be generated by a free-running oscillator driven by a crystal, an LC-tuned circuit, or an external clock source. Parameters of a clock signal may include maximum and minimum allowable clock frequencies, tolerances on high and low voltage levels, maximum rise and fall times on waveform edges, pulse-width tolerance for a non-square wave, and the timing relationship between clock phases if two-clock phase signals are needed.
High-speed electronic circuits are particularly susceptible to generating and radiating electromagnetic interference (EMI). Accordingly, many regulatory agencies, such as the Federal Communication Commission (FCC) and the Comite International Special Des Perturbations Radioelectriques (CISPR), have established maximum allowable EMI emission standards for electronic equipments, and promulgate guidelines concerning measurement equipment and techniques for determining EMI compliance.
The spectral components of the EMI emissions typically have peak amplitudes at harmonics of the fundamental frequency of a clock circuit. In order to comply with the above-mentioned governmental limits on EMI emissions, costly suppression measures or extensive shielding can be utilized. Other approaches for reducing EMI emissions include careful routing of signal traces on printed circuit boards to minimize loops and other potentially radiating structures. However, EMI emissions are made worse at higher clock speeds.
Consequently, it would be desirable to provide an improved method and apparatus for generating high-speed clock signals having relatively low EMI emissions.